Aer Ryan
Your rumour is simply that... The easyJet HQ or hangar 89 as it is officially known was secured as part of Luton being classed as an economic regeneration zone which helped secure big discounts in land/taxes. Several sites around Luton were considered.
If and when the time comes that the HQ is too small in size or the land is needed within the CTA at Luton then easyJet will move to another site within the surrounding area. The other option that was considered to H89 when easyLand became to small was a large industrial unit next to Luton Parkway station that is now home of the printing site for Express newspapers.
Land around West Sussex is more expensive than up in Bedfordshire. easyJet had the option of taking on The Beehive when it took over GB Airways and did not. Of course there are some functions already at Gatwick and some functions will grow there but an airline's hq has no need to be at it's biggest hub... Ryanair's, Flybe, Thomson, Monarch, Wizzair seem to agree with this idea.
The whole idea of easyJet supporting Heathrow's expansion is to protect it's slots and market share at Gatwick. If a third runway is built at Gatwick then easyJet are the biggest losers with increased competition and the value of their slots diminish. If Heathrow is the winner then easyJet protects its position as number 1 in Gatwick and gets to cherry pick routes at Heathrow probably with flights in from other bases.